1. Field of the Invention
The present invention relates to a radiation imaging apparatus using a portable solid-state imaging device configured to allow a grid to be mounted outside the housing.
2. Description of the Related Art
Conventionally, apparatuses which obtain radiographic images of objects by irradiating the objects with radiation and detecting the intensity distributions of radiation transmitted through the objects have been widely and generally used in the fields of industrial nondestructive testing and medical diagnosis. As a general method for such radiography, a film/screen method using radiation is available. This is the method of performing radiography by using a combination of a photosensitive film and a fluorescent having sensitivity to radiation.
In this method, rare-earth fluorescent sheets which emit light upon application of radiation are held in tight contact with the two surfaces of a photosensitive film. The fluorescent converts radiation transmitted through an object into visible light. The method then develops, by chemical treatment, the latent image formed on the photosensitive film by making it capture this visible light, thereby visualizing the image.
The recent advances in digital technology have popularized the scheme of obtaining high-quality radiographic images by converting radiographic images into electrical signals, performing image processing for the obtained electrical signals, and then reproducing the resultant information as visible images on a CRT or the like. As such a method, there has been proposed a radiographic image recording/reproduction system which temporarily stores a transmission image of radiation as a latent image in a fluorescent, photoelectrically reads out the latent image by irradiating the fluorescent with exciting light such as a laser beam, and then outputs the readout image as a visible image. In addition, with the recent advances in semiconductor process technology, there has been developed an apparatus for capturing a radiographic image in the same manner as described above by using a semiconductor sensor.
These systems have very wide dynamic ranges as compared with conventional radiographic systems using photosensitive films, and can obtain radiographic images which are robust against the influences of variations in the amount of radiation exposure. At the same time, unlike the conventional photosensitive film scheme, this method need not perform any chemical treatment and can instantly obtain an output image.
FIG. 7 is a view showing the arrangement of a radiation imaging system using the above semiconductor sensor. A radiation imaging apparatus 103 mounted on a radiographic stand 106 includes a solid-state imaging device 104 having a detection surface on which a plurality of photoelectric conversion elements are two-dimensionally arranged.
A radiation generator (X-ray tube) 101 emits radiation to irradiate an object 102. The solid-state imaging device 104 then images the radiation transmitted through the object 102, and converts it into visible light through the fluorescent. A control unit 107 reads out the electrical signal output from the solid-state imaging device 104, performs digital image processing for the signal, and then displays the resultant information as a radiographic image of the object 102 on a monitor 108.
The radiation imaging apparatus 103 as an imaging unit incorporates an anti-scatter grid (to be referred to as a grid hereinafter) 105. The grid is designed to remove scattered X-rays generated inside the object (e.g., a human body) 102 upon X-ray irradiation, and is used to improve the contrast of an X-ray image. This apparatus performs radiography with the grid 105 being disposed between the X-ray tube 101 and a detector such as a film. Such grids are defined as JIS Z 4910 anti-scatter grids, which will be briefly described below.
FIG. 8 is a schematic sectional view of the grid described above. X-rays are applied from a direction A on the left side of FIG. 8. The grid is formed by alternately stacking foils 201 made of a material having a high X-ray absorptance and intermediate materials 202 having a low X-ray absorptance. In general, lead is used for the foils 201 having a high absorptance, and aluminum, paper, wood, synthetic resin, carbon fiber reinforced resin, or the like is used for the intermediate materials 202 having a low X-ray absorptance. The outer surface of this multilayered structure is covered by, for example, an aluminum or carbon fiber reinforced resin cover.
In many cases, the above grid is a focused grid including a foil represented by a foil 201a which is located at a central portion immediately below the X-ray source and is perpendicular to the cover and foils 201b which gradually tilt in the direction of the light source toward the fringes. When a focused grid is to be used, it is necessary to perform radiography upon adjusting the distance between the grid and the light source and their centers. A grid without any tilting of foils is also available, which is called a parallel grid. Such grids differ in the property of attenuating transmitted X-rays depending on the density or geometrical shape of foils. A grid with optimal specifications is selected in accordance with radiography. In particular, the solid-state imaging device 104 described above is generally selected so as to prevent the pixel size from interfering with the intervals between grid foils in terms of frequency.
An imaging apparatus of this type has been installed and used in a radiation room. Recently, a portable imaging apparatus (also called an electronic cassette) has also been provided to allow quicker radiography of regions in a wider range.
Such an electronic cassette is required to be low in profile and lightweight and have high mechanical strength. In cassette radiography, a person as an object may be rested on the cassette. In addition, since the electronic cassette is portable, a shock may act on the cassette if it is dropped or collides with something. As compared with conventional stationary imaging units, therefore, it is necessary to greatly improve the resistance of such electronic cassettes in terms of mechanical strength.
A cassette can be applied to various regions, and hence the grid is preferably configured to be easily attached/detached depending on a region to be radiographed. Therefore, a grid which can be mounted outside an imaging unit has been proposed as disclosed in Japanese Patent Laid-Open No. 2004-177251. This grid is mounted on a metal frame component to secure its mechanical strength.
As a grid mounted outside an imaging unit like that described above, a grid having the following characteristics has been provided. The first characteristic is that a metal frame member is mounted on the grid body to protect it in terms of mechanical strength. The second characteristic is associated with the pixel array of the imaging unit and the relative angle of the grid lattice.
Unlike film radiography, digital imaging has the merit of reducing, by image processing, streaks appearing on an image when the foils of the grid are captured on it. For this reason, the relative angle preferably falls within the computational tolerance of image processing. Therefore, in order to make the relative angle fall within the tolerance range, the grid is mounted on the imaging unit with a side wall being provided on a frame member for positional restriction for the imaging unit. In addition, some grids have a buffer member mounted on a side wall to prevent the operator from being injured if the grid is accidentally dropped or to prevent the grid from being damaged during transport.
Consider a case in which a portable X-ray imaging apparatus 111 is used to radiograph a side surface of a head portion 110 on a table 114, as shown in FIG. 9. In this case, since a distance L from the outer shape of a grid fixing frame 113 to an effective imaging area 112 of the imaging unit 111 is large, it is necessary to use a tool for applying some correction for an offset relative to an object. That is, such radiography accompanies cumbersome operation.
Studies have been focused on the imaging unit to meet the requirement for a reduction in weight. In practice, however, it is necessary to implement weight reduction, including a reduction in the weight of the grid.